1. Field of the Invention
The present invention relates to surface liquefaction of mined coal by means of hot gases generated via underground gasification of carbonaceous material. Following liquefaction, the resulting char is burned in boilers to generate steam for use in the underground coal gasification plant.
2. Setting of the Invention
There are two well-known methods for mining coal. First, coal deposits at or near the surface are mined by strip mining which involves removing the overburden and thereafter removing the coal. Strip mining may cause serious environmental degradation of the surface and undersurface aquifers. Further, strong acidic components can leech the material exposed after the overburden is removed. Ecological restoration of land is expensive and may not be fully successful. Strip mining has had strict ecological and environmental controls placed upon it.
Secondly, where coal deposits are at considerable depths, 500 ft or greater, conventional shaft mining techniques must be resorted to. Mining of coal deposits too deep to be stripped of overburden involves large amounts of labor. Shaft coal mining is accomplished by a high incidence of accidents caused by mine collapses and explosions. Furthermore, deep mining of coal is inefficient in that only half the coal is extracted and that most of it is not mined at all because the seams are either too thin or too deep to permit economic working.
The present invention reduces the amount of mining required by combining a mining operation with underground coal gasification.
Underground gasification has proved to be useful to obtain energy from coal deposits. Coal gasification involves a high temperature reaction of carbonaceous material and water to produce primarily hydrogen, carbon monoxide and carbon dioxide. The required heat for the reaction can be provided by partial combustion of the coal and/or gasification products. The combustion method utilized can be forward combustion. Coal gasification methods require the drilling of a production well and an adjacent injection well through which oxygen and steam are injected into the coal seam. A flame or combustion front is initiated adjacent the wellbore face of the injection well and is advanced through the coal seam. The combustion front provides the heat energy necessary to gasify the coal. The combustion and gasification products (henceforth referred to as synthesis gas) are thereafter removed through the production well for use at the surface.
An object of the present invention is to provide a process for utilizing underground coal gasification to recover coal values and the use of the heat available in the produced synthesis gas to pyrolyze mined coal thereby recovering values therefrom. The resulting char from the pyrolysis step is available for steam generation in boilers.
Other objects and advantages of the present invention will be apparent to one skilled in the art upon reading this disclosure.